This invention relates to magnesium oxide based compositions in the form of attrition-resistant fluidizable microspheres which are circulated with microspheres of zeolitic fluidizable cracking catalysts. The magnesium oxide based particles minimize or prevent poisoning and deactivation of the zeolitic cracking catalysts by vanadium contained in oil feedstock used in the catalytic cracking process. The invention relates also to processes for producing such compositions by spray drying a slurry containing magnesium oxide, kaolin clay and in situ formed inorganic cement.
Poisoning and deactivation of catalyst in FCC (fluid catalytic cracking) by vanadium in the oil feedstock is one of the most prominent problems faced by operators of oil refineries. Patents including disclosure of the use of alkaline earth compounds, including magnesium oxide, to mitigate the effects of vanadium include U.S. Pat. Nos. 4,465,779, 4,549,548, 4,944,865, WO 82/00105, GB 218314A, EP-A-020151 and EP-A-0189267. In some of these references, the magnesium oxide is contained in discrete particles, separate from the particles of zeolite cracking catalyst. EP-A-270,211 discloses discrete particles in which magnesium is present as a crystalline magnesium silicate, preferably fosterite. The material containing crystalline magnesium silicate may be produced by spray during a slurry containing magnesium carbonate and kaolin clay, followed by high temperature calcination to react the magnesium with silica in the clay, forming the crystalline magnesium silicate. Other disclosures of crystalline magnesium containing silicates (clays such as sepiolite) appear in U.S. Pat. No. 4,549,548 supra.
Efforts to develop products and processing modifications to mitigate vanadium passivation are by no means limited to the use of alkaline earth material. To the best of our knowledge, however, no magnesium based additive has enjoyed widespread commercial success. Certain perovskites such as barium titanate are employed commercially. Perovskites are expensive. Perovskites are not considered to be very effective in reducing SOx emissions in regenerator flue gas. Alkaline earth material, especially magnesium oxide, offers the additional benefit of reducing Sox in regenerator flue gas from cracking units. See, for example, WO 82/00105GB (supra).
There is strong motivation to utilize the inherent vanadium binding and Sox capturing capacity of magnesium oxide in FCC operations utilizing feedstocks having a high content of vanadium. References cited above give some indication of past efforts to produce magnesium oxide based vanadium passivating particles adapted for co-circulating with zeolite cracking catalysts. Commercial success has not measured up to the motivation. One primary challenge was to provide a metals passivator in a physical form of particles sufficiently attrition-resistant for use in FCC, while maintaining the magnesium in most reactive form (oxide). This problem was addressed in EP-A-270,211. The solution proposed in that patent application resulted in particles that were attrition-resistant in fresh form but lost hardness when subjected to steaming in testing.
Magnesium oxide without a binder/matrix is unsuitable for use in an FCC unit when it must be circulated through the reactor and regenerator of an FCC unit along with cracking catalyst particles. This is because particles of magnesium oxide readily break down into a powder when subjected to attritive forces. Note that in one of the earliest proposals to use magnesia in an FCC unit to combat SOx (U.S. Pat. No. 3,699,037), the material was circulated in the regenerator to bind SOx. The magnesia attrited during such use, eventually to be withdrawn from the regenerator with flue gas without circulating in the cracker, as would be required to achieve vanadium passivation. Because of the friable nature of magnesium oxide particles, the material did not circulate with the catalyst during the FCC cycle.
Numerous patents, including several of those cited above, disclose formulations based on composites of magnesia with kaolin clay. Kaolin clay is a widely used matrix component for cracking catalyst because it is inexpensive and has potential binding properties. Also, it is relatively catalytically inert in calcined form and is a prime candidate as a matrix/diluent for a vanadium passivator based on magnesia, wherein catalytic activity is not desired. An advantage of using kaolin clay as a matrix/diluent is that it can readily be formed into substantially catalytically inert particles by forming a dispersed concentrated fluid slurry to form microspheres, followed by spray drying. When dried, especially when calcined, kaolin also serves as a binding function.
Several of the references noted above provide examples of MgO/kaolin microspheres prepared by means including spray drying, but they do not disclose the composition of the feed slurry to the spray dryer. They do not provide information about attrition-resistance. There is no indication that the inventors were concerned with attrition-resistance or steam stability of the products. In the case of WO 82/00105GH, the matrix was a mixture of kaolin and silica-alumina gel, a conventional matrix for zeolite crystals in an active cracking catalyst. Silica-alumina is a material known to possess catalytic activity.
EP-A-270211, supra, refers to difficulties encountered in achieving attrition-resistance by mixing magnesia with kaolin clay, spray drying and calcination.
Those skilled in the art of handling clays are aware that introduction of magnesium ions into clay slurries causes the slurry to flocculate and thicken. This has been used with benefit in the formulation of various clay-based drilling mud. However, flocculation and thickening causes formidable problems in producing magnesia/kaolin clay products useful for FCC wherein particles of appreciable magnesium oxide content are produced in spray dryers. It is a simple matter to provide a dispersed kaolin slurry that is sufficiently fluid at a high enough concentration (e.g., 50% solids) to produce coherent microspheres. However, if kaolin is spray dried at low solids, e.g., 10%, the microspheres will fall apart before they can be hardened by calcination. If magnesium is added to such a high solids fluid dispersed slurry of kaolin in more than a trace amount, the slurry will flocculate and thicken. If enough magnesium ions are introduced, a solid gel forms and the slurry cannot be formed into microspheres by spray drying using known technology. Addition of magnesium oxide to a kaolin slurry in amount sufficient to produce spray dried particles having a sufficiently high MgO content for effective vanadium passivation will result in a slurry that cannot be spray dried in continuous commercial spray drying equipment. This problem plagued the inventors of the subject patent application in their pursuit of developing attrition-resistance spray dried microspheres containing magnesia with a clay diluent which meet the criteria for a good vanadium trap: attrition-resistance; high capacity for vanadium trapping; good vanadium passivation; and very high trapping efficiency (i.e., fast vanadium uptake).
To produce such particles it was necessary to overcome the difficulty caused by flocculation of a dispersed slurry of kaolin clay by the incorporation magnesium ions, resulting in thickening or even gelation of the slurry and, ultimately, the inability to formulate a slurry of sufficiently high solids content to produce attrition-resistant spray dried microspheres. The need to control flocculation and thickening to achieve hardness was counterbalanced by the need to product microspheres that were sufficiently porous to function as an effective magnesium passivator.